Kinetic deposition of particles

ABSTRACT

A method of coating a substrate in which a mass of particulate material is formed in situ or disposed between a pair of electrodes which produce a spark discharge to create a shock wave carrying the particles onto and bonding them to the substrate. The particles are activated by their presence within the discharge.

United States Patent Inoue [451 May 16, 1972 [54] KINETIC DEPOSITION OFPARTICLES 52 us. c1 ..219/76 51] Int. Cl ..B23k 9/04 [72] Inventor.Klyoshl Inoue, 182, 3-chome, Tamagawayoga Machi, setagaykkuy [58] FieldofSearch ..219/76, I21, 149, 75/208, 226, Tokyojapan v ll7/93.l, 105,105.2 [22] Filed: July 29, I970 [56] References Cited [211 APPL 641104UNlTED STATES PATENTS Related Application Data 3,371,404 3/1968 Lemelson..29 421 E [63] Continuation of s 574 05 Aug. 22 19 2,714,563 8/1955Poorman et al ..ll7/l05 abandoned, which is a continuation-in-part ofSer. Nos, 311,061, Sept. 24, 1963, Pat. N0. 3,267,710, and Prmmry TruheSer. No. 508,487, Nov. 18, 1965, Pat. No. 3,512,384, si nt Bu me -J. G.Smi h which is a continuation-in-part of Ser. No. 41,080, July 6, 1960,Pat. No. 3,232,085. [57] ABSTRACT [30] Foreign Application Priority DataA method of coating a substrate in which a mass of particulate materialis formed in situ or disposed between a pair of elec- June ll, 1966Japan ..41/37635 trodes which produce a spark discharge to create ashock June 1 1966 P wave carrying the particles onto and bonding them tothe suby 1 1 1966 Japan strate. The particles are activated by theirpresence within the Aug. 19, 1966 Japan ..41/54709 di h Aug. 19, 1966Japan ..41/54710 25 Claims, 16 Drawing Figures 5 I4 I I0 V X 2/ i 27 2423 c kg J J 20 6 i ll -INSULATED 17 12 1s I 22 I 2| L,1 g 37 JvAC. E W I5 Patented May 16, 1972 3,663,788

.6 meets-Sheet 2 KIYOSHI INOUE INVENTOR.

Patented May 16, 1972 3,663,788

.6 Sheets-Sheet 5 F I G. 7

KIYOSHI mous INVENTOR.

Patented May 16, 1972 .6 Sheets-Sheet 4 Fig.1O

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Kiyoshi Inoue INVENTORL BY A Afforneg Patented May 1 1-9 2 3,663,788

. .e Sheets-Sheet e ass FIG. 13A] TO PULSE 2 2! SOURCES FIG- I3 I-K|YOSH| INOUE INVENTOR.

. B (I, 206 Y Attorney KINETIC DEPOSITION OF PARTICLES CROSS-REFERENCETO RELATED APPLICATIONS This application is a continuation of mycopending application Ser. No. 574,056 filed August 22 1966, nowabandoned; said abandoned application is acontinuation-in-part of mycopending application Ser. No. 311,061, filed Sept. 24, 1963 (now U.S.Pat. No. 3,267,710) and application Ser. No. 508,487,-filed Nov. 18,1965(now U.S. Pat. No. 3,512,384 issued May 19, 1970) as acontinuation-in-part of application Ser. No. 41,080 of July 6, 1960, nowU.S. Pat. No. 3,232,085, issued Feb. 1, 1966.

FIELD OF THE INVENTION The present invention is directed to improvementsin metalforming, bonding and deposition with the aid of impulsive forceswhereby pulverulent materials and especially comminuted metallicsubstances can be applied to a substrate.

BACKGROUND OF THE INVENTION In application Ser. No. 311,061 (U.S. Pat.No. 3,267,710),I describe and claim a method of shaping metals andbonding metals to other bodies by impulsive action generated at least inpart by an electrical discharge in a fluid. The principle of this systemis that an electric discharge in a fluid, e.g., a viscous medium such asa liquid or a gas, produces a shock wave which can be transmitted to ametallic body in order to transfer the kinetic energy of the shock waveto this body and apply it against a die or use the energy to compact amass of particles. In that application, I have pointed out that aparticulate material can be converted into a coherent mass (i.e.,particles of metal bonded to other metallic bodies, namely, otherparticles) with the aid of an impulsive shock generated by effecting aspark discharge in a closed vessel containing a liquid medium inforce-wave transmitting relationship to the bodies whose kinetic energyis used to produce the bonding action. Surprisingly, the relativelyinstantaneous or momentary development of a shock within a body of fluidis found to give rise to a Shock wave whose velocity and energy is suchthat, upon transfer of this energy to the particles, the particlesand/or a body against which they are applied will deform at a velocitysufiicient to permit bonding between them.

While the fusion of particles of a metallic powder to form a coherentstructure is not new, the conventional methods of accomplishing thistask have heretofore involved the provision of sufficient kinetic energyin the form of heat to effect a thermal fusion at the interface of theparticles. The disadvantages of these conventional methods are wellknown and need not be elucidated here, except to note that they requirerelatively high temperatures and pressures.

In my U.S. Pat. No. 3,250,892 issued May 10, 1966 and in the copendingapplications Ser. No. 326,837, filed Nov. 29, 1963 now U.S. Pat. No.3,317,705 of May 2, 1967), and Ser. No. 356,714, filed Apr. 2, 1964,(now U.S. Pat. No. 3,340,052 of Sept. 5, 1967), I disclose a method ofsintering discrete particles together wherein the high pressures andextreme thermal conditions of conventional sintering methods can beavoided. In this method, electric-spark discharge is employed to providean impulsive force which is more or less instantaneous in nature andserves to drive the particles into contact with one another at highpressures, while a discharge is propagated among the particles to causebridging, an operation which appears to result from the transfer by thedischarge of material from one particle to another. This system ishighly suitable when a spark discharge can be developed between twoelectrode surfaces having the particles disposed therebetween; however,the power necessary for the coating of large-surface continuous bodieswith the particles is not as readily available. Even prior to theaforementioned applications relating to the sintering of discreteparticles to one another, 1 disclosed in my U.S. Pat. Nos. 3,232,085 and3,232,086 issued Feb. 1, 1966 spark discharge with the discharge beingpropagated through a fluid medium. It has also been proposed by others,as described in U.S. Pat. No. 3,137,937 for example, to clad acontinuous metallic substrate with another metal layer by spacedlyjuxtaposing the layer and the substrate by relatively small distancesand propagating an explosion along the surface of the layer such thatthe latter is effectively rolled onto the substrate parallel to thelayer. According to this system, a sheet explosive is applied over thefull surface of the layer at which bonding is required and propagationis effected parallel to the explosive layer, the substrate and thecoating layer. Such arrangements are not, however, practical for thecoating of substrates with pulverulent materials. 1

OBJECTS OF THE INVENTION It is the principal object of the presentinvention to provide an improved method of coating metallic substratewith pulverulent materials in such manner as to render the coating layerhighly adherent while obviating the disadvantages of earlier coatingprocesses.

Still another object of this invention is to provide an improvedapparatus for the coating of metallic bodies with pulverulent materials.

A further object is to provide a method of and an apparatus for formingstrong bonds with metallic and nonmetallic substrates and conductive andnonconductive particles.

Yet a further object of my invention is the provision of a method ofcoating relatively soft bodies (e.g., of steel or the like) withrelatively hard surfacing layers (e.g., tungsten carbide) in such manneras to form a strong bond between the coating material and the substrate.

SUMMARY OF THE INVENTION These objects and others which will becomeapparent hereinafter are attained in accordance with the presentinvention, by a method for the deposition of layers upon a substratewhich is an extension of the principles set forth in my earlierapplications and can involve the exploitation of the large amount ofkinetic energy produced in an impulsive discharge.

Thus, the present invention resides in a system for the coating ofmetallic substrates with pulverulent materials which comprises the stepsof juxtaposing a source of a detonationtype impulsive wave with asurface of the body to be coated and disposing between the body and thesource, a mass of a pulverulent material, which can be harder than thesubstrate and even nonbondable thereto by conventional thermal methods,the powder mass being disposed in the proximity of the detonationsource; the production of the detonation wave by the source drives theparticles onto the substrate with suflicient velocity as to lodge thenthereon with a firm bond between the layer and the substrate.

I have found also that excellent results are obtained with respect tothe bonding of particles of a hard-facing material of the type set forthabove or alloy steels to metallic substrates,

' synthetic-resin substrates or the like if the discharge source isspaced from the mass of powder which, in turn, is disposed between thesource and the surface of the substrate to be coated. Thus, it isadvisable to support a layer of powder upon a frangible foil, film,sleeve or sheet juxtaposed with the surface to be coated whereby therupturable diaphragm supporting the particle layer can separate thedischarge chamber from the workpiece chamber. The latter, of course, canbe vented to the atmosphere to prevent the development of pressuresresisting kinetic movement of the particles into bonding engagement withthe workpiece and the venting means preferably includes a sound-dampingmufiler. In accordance with this aspect of the invention, it has beenfound that a significant improvement in the degree with which theparticles bond to the substrate can be attained when the particles arelocated in the discharge compartment although, in accordance with amodification, the particle layer may be carried by the frangiblediaphragm along its surface juxtaposed with the workpiece and remotefrom the discharge chamber. Furthermore, the

frangible diaphragm preferably constitutes the counter-electrode for thedischarge system and this arrangement has been observed to giveexcellent results when broader surfaces of a workpiece are to be coatedbecause the particle cloud is then driven against the workpiece with akinetic energy that, while greater in the region of local discharge, maybe considered substantially uniform over the entire cross-section of thecloud for practical purposes. The discharge electrode is a needle spacedfrom the frangible diaphragm, which can be an aluminum foil or composedof any other suitable metal, the needle extending perpendicularlythereto.

In accordance with this aspect of the invention, it has been founddesirable to provide the discharge chamber as a discharge gun whosebarrel is trained upon the workpiece and receives at an intermediatelocation a mass of particles to be propelled thereagainst. In ahorizontal position of this barrel, the particles can be introducedsubstantially continuously between the discharge chamber and the mouthof the barrel, while a rapid train of pulses is supplied across theelectrodes so that a sequence of discharges results in an intermittentbut high-rate propulsion of the particles against the workpiece surface.In vertical positions of the barrel, it has been found most practicaland highly advantageous to make use of the foil-type support andcounter-electrode set forth above. The needle electrode is bestconstituted of aluminum although zirconium, magnesium and copper havebeen found to rank in that order with reference to the kinetic energytransferred to the particles. Correspondingly, the foils should beconstituted of these metals in the order stated. While I do not wish tobe bound by any theory as to the reasons why the bonding efiect and thekinetic-energy characteristics of the metal particles are determined inpart by the metal from which the needle electrode and foil areconstituted, it will be noted that rupture of the foil, partialvaporization of the electrode material during the discharge and the highvelocity of the shock propagation permit any atoms or molecules of theelectrode material to act in force-transmitting relationship with theparticles to be deposited.

According to another aspect of this invention, the effective kineticenergy of the particles and the strength with which the particles bondto the surface, whether it be to ametallic or a synthetic-resinsubstrate, can be augmented by providing means for heating the particlesto a temperature less than their fusion point prior to their propulsionagainst the substrate. Such heating means can include the passage of aheating current through the mass of particles in advance of thedischarge, the use of external electric heating means or some equivalentsource or, most advantageously, the mixing with the particles to bedeposited of a substance adapted to react with these particlesexothermically. Thus, aluminum oxide can be provided together with areducing agent (e.g., iron) or magnesium with iron oxide inthermite-type reactants capable of evolving sufficient heat to improvethe bonding effectiveness. It has been observed that thermite-typereactants tend to remain in a quiescient state until the generation of aspark discharge and that the reaction may occur slightly before orconcurrently with dispersion of the particles so that they are heatedwithout material interparticle fusion until they accumulate again as alayer upon the surface of the substrate.

Another factor entering into the improved bonding action appears to bethe effect of the spark discharge in stripping oxide films from theparticles and/or the substrate even though analysis shows no materialoxide layer to be present. I

have observed that practically all metallic particles have an oxide filmwhich resists interparticle bonding as well as particle-to-substratebonding to such an extent that high temperatures have hitherto beenrequired to effect suitable bonding strength. While I have pointed outin my prior applications relating to the spark sintering of particlesthat the discharge between adjoining particles tends to strip such oxidefilms therefrom, it has now been discovered that an electric dischargein the region of the particles and not necessarily involving themdirectly may have a similar effect. It will be noted that variousmethods of initiating the discharge can be employed although two havebeen found most practical in combination with the foil-and-needlearrangement set forth earlier. In such an arrangement, the needle caneither be advanced toward the foil to reduce the discharge gap (wherebyan external source of pulses is not required), or the ionizationconditions within the discharge compartment may be suddenly modified.This can be done effectively by directing a stream of compressed airinto the chamber to stir up a cloud of the conductive particles to bedeposited, whereupon a breakdown is produced. The discharge-propagatedparticles form a layer of considerable uniformity and highly effectivesurface area. Thus, when tantalum or titanium are deposited upon analuminum foil, excellent capacitor plates are obtained. When gold oraluminum are formed as films upon a silicon wafer, excellentsemiconductive materials are produced. Photoconductive cells can be madereadily by deposition of lead sulfide and cadmium sulfide upon suitablesubstrates.

According to an important feature of this invention, the detonationsource includes a pair of electrode elements adapted to define betweenthem an electric-discharge gap, the pulverulent material being disposedin close proximity to the gap and advantageously surrounding it. The gapcan be temporarily bridged by a fusible element which is disintegratedupon discharge of a high-energy pulse across the gap and serves tolengthen the effective time of discharge as a consequence of the delayedopeningof the gap. As described in U.S. Pat. Nos. 3,232,085 and3,232,086, a shock wave is propagated through the fluid medium betweenthe source and the substrate of a high velocity sufficient to defonn aworkpiece, but here used to bond particles to a workpiece surface. Ihave found that, earlier teachings regarding the necessity that thepropagation of the explosive wave be parallel to the surfacenotwithstanding, excellent results are obtained when the particles aredisposed in the immediate region of the detonation source and theviscosity of the surrounding medium is reduced (e.g., by evacuating theregion of the detonation). Powders having a particle size rangingbetween 0.2 microns and 0.2 mm are suitable for the practice of thepresent invention while the distance of the source from the substratevaries as a function of the applied energy. When a discharge of 3,000joules is employed, for example, in an atmospheric medium but at areduced pressure of 10 /mm Hg, the detonation source should be about 15mm from the workpiece. It has been found that the shock wave and,therefore, the particles accelerated thereby should attain a minimumvelocity of about 10 m/sec, although the effectiveness of bonding fallsoff sharply below m/sec and velocities as high as 10 km/sec aresuitable. The particles may be composed of relatively hard metals ormetallic substances among which the most preferable are tungstencarbide, titanium carbide, boron carbide, nickel, copper and iron.Almost any suitable substrate may be employed (e.g., steel, nickel,copper and its alloys, synthetic resin, etc.). Nonmetallic particles ofdiamond, silicon carbide, aluminum nitride, boron nitride, lead sulfide,cadmium sulfide and the like can also be readily bonded to thesesubstrates. Other useful powders include A1 0 SiO PbO, and ZnO, all ofwhich can react with oxidizable metals in exothermic processes asmentioned above.

According to yet another feature of this invention, the shock wave,entraining the particles at high velocity in the direction of theworkpiece, is concentrated in this direction by suitable deflectionmeans (e.g., mechanical reflectors or electromagnetic devices takingadvantage of the fact that the particles are magnetically permeable andthe detonation wave generally contains ionized or charged particlesproduced by the detonation). It has been found to be advantageous toprovide a high-frequency electric field across the region in whichpropagation takes place.

DESCRIPTION OF THE DRAWING The above and other objects, features andadvantages of the present invention will become more readily apparentfrom the following description, reference being made to the accompanyingdrawing in which:

FIG. 1 is a vertical cross-sectional view, in diagrammatic form, showingan apparatus for depositing particulate aterials upon a workpiece;

FIG. 2 is a diagrammatic sectional view showing a modified apparatus;

FIG. 3 is a view similar to FIG. 1 wherein, however, a highfrequencyfield is employed concurrently;

FIG. 4 shows a modified circuit arrangement for the system of FIG. 3;

FIG. 5 is a fragmentary elevational view showing still anothermodification of the detonation source;

FIG. 6 is a view similar to FIG. 5, illustrating the use of a fusiblewire within the body of particulate material;

FIG. 7 is a transverse cross-sectional view through yet anothermodification of the source, provided with reflector means; I

FIG. 8 is a diagrammatic elevational view, partly in section, showing amagnetic-deflection arrangement;

FIG. 9 is an illustration of the arrangement of the present invention asused in Example I; v

FIGS. 10 and 11 are photomicrographs with enlargements of I00 and 800times, respectively, showing the junction between a depositedparticulate material and a substrate;

FIG. 12 is an axial cross-sectional view of an apparatus embodying theprinciples of my present invention in accordance with a modificationthereof;

FIG. 13 is an axial cross-sectional view of a vertical-barrelparticle-deposition gun somewhat in diagrammatic form;

FIG. 13A represents a modification of the latter structure;

FIG. 14 is an axial cross-sectional view of another gun arrangement inwhich the particles are projected downwardly; and

FIG. 15 is a cross-sectional view through a system in which theparticles are bonded together by the impulsive energy.

SPECIFIC DESCRIPTION AND EXAMPLES In FIG. 1, I show a housing 10 whosecompartments l1 and 12 on opposite sides of a workpiece 13 containelectrode supports 14 and 15, respectively. The supports 14, 15 arehorizontally displaceable within respective bearings l6, 17 against theforce of restoring spring 18, and carry sparkdischarge electrodes 19,via locking screws 21, 22. The electrodes 19 and 20 define a dischargegap 23 between them and are surrounded by a mass 24 of particles to becoated onto the workpiece 13. The latter is disposed below thedetonation source constituted by the electrodes. A capacitor 25 isbridged across the electrode holders 14, 15 together with aparametrically energizable capacitor 26, the parametric transformerbeing constituted by a solenoid coil 27 surrounding the arrnature 28rigid with the electrode holder 15. A full-wave rectifier bridge 29 isconnected across the secondary winding of a power transformer 30supplied with current by the a.c. source 31 and energizes the low-turnportion of the solenoid 27. A servomotor 33 is provided to sweep thedetonation source along the surfaces of the workpiece l3 perpendicularlyto the plane of the drawing. Any conventional servodevice is suitablefor this purpose.

When alternating current is applied at 31 to transformer 30, thecapacitors 25 and 26 are charged via the respective windings of thesolenoid 27. When a sufficiently high potential is attained for abreakdown of the gap 23, a spark jumps between the electrodes 19 and 20and the resulting impulsive force or detonation drives the pulverulentmaterial into the workpiece 23. The discharge of capacitor 25, forexample, draws current through the solenoid 27 and effects a lateraloscillation of the assembly against the force of spring 18, capacitor 26further sustaining the discharge and promoting oscillation whilelengthening the duration of the detonation. This oscillation increasesthe area of the workpiece 13 swept by the detonation wave.

In FIG. 2, I show a modification wherein the electrode 32, 33,surrounded by the pulverulent material 34 is juxtaposed with a furtherbody 35 in line with the gap. Either the workpiece 36 or this furtherbody 35 can constitute a reflector redirecting the shock wave and makingmaximum use of the discharge. Similarly, either one of these twoelements can be the workpiece upon which the pulverulent material isdeposited. A partition 37 (dot-dash lines in FIG. 1) can close thecompartment 38 in the housing and enable it to be evacuated forlow-pressure deposition. Additionally, nonoxidizing gases (e.g.,nitrogen) can be employed as the medium within the compartment.

In the system of FIG. 3, the vessel 40 is closed by a seal 41 and acover 42 while a suction pump 43 serves to reduce the pressure withinthe vessel. The vessel contains a pair of spaced-apart plates 44, 45,constituting the workpiece and disposed on opposite sides of adetonation source constituted by a pair of electrodes 46, 47 urgedinwardly by respective springs 48. The mass of particles is hereconstituted as a relatively narrow tube 49 of powder through theinterior of which a discharge can be initiated upon closure of switch50. Here again the discharge source extends parallel to the surfaces ofthe workpiece 44, 45 to be coated so that the shock wave is generallytransverse to these surfaces. A direct-current bridge 51 charges thecapacitor 52 via a surge-suppressing inductance 53 upon energization bythe power transformer 54 and the a.c. source 55. A high-frequencydirect-current source 56 connected with the plates 44, 45 facilitatesthe deposition of the powder and improves the structure of the deposit.The high-frequency signal can range between about I kc/sec to about 10mc/sec.

In FIG. 4, a similar arrangement is shown, except that the a.c. orhigh-frequency generator 56 is provided by a pair of resonant networks57 formed on opposite sides of the inductance center tap and connectedwith the plates 44, 45. Since these electrodes are now at the samepolarity, the highfrequency field produced upon discharge across the gapis applied between the body of powder and the plates. The shock waves ofFIGS. 3 and 4 can be augmented by disposing a fusi= ble wire 58 parallelto the powder rod 59 between the electrodes 60, 61 (FIG. 5); the systemis otherwise identical with that of FIG. 3. The ganged switches 62, 63fulfill the function of switch 50 of FIG. 3 and are adapted to dischargethe capacitor 52 joined through the spark gap and the fusible wire whichis disposed on the side of the rod 59 opposite the workpiece. The shockwaves from wire fusion and are discharge thus supplement one another indriving the powder across the gap. This system is an improvement overthe mere spark discharge with respect to the quantity of powder affixedto the substrate per unit of power consumed.

A still more efiicient system results when the fusible wire issurrounded by the mass of particles as shown in FIG. 6. Here, thefusible wire 70 is surrounded by a tubular mass 71 of the particles tobe bonded to the substrate 72. A capacitor 73 can be discharged throughthe wire 70 to disintegrate it when switch 74 is closed, a battery 75being provided to charge the capacitor 73. Again means can be providedfor oscillating the detonation source with respect to the workpiece orotherwise displacing the source and workpiece relatively to obtainmaximum powder coverage. The system of FIG. 6 can, moreover, be providedwith a downwardly concave (e.g., parabolic) reflector 76 with the rod 70disposed at the focus. An excellent distribution of the powder isobtained in such a system while the depositing efficiency is improved(FIG. 7). When the mechanical deflection is replaced by anelectromagnetic deflection of the shock wave, the magnetic means 77(FIG. 8) is oriented so that its field is applied transversely to thedetonation source 78 and the workpiece 79 but in the plane of thedetonation source. Highly suitable results are obtained when thedetonation force includes a fusible wire 80 connected in series with theelectromagnet 77 across the capacitor 81. A switch 82 permits dischargeof the capacitor through the series arrangement and allows charging ofthe capacitor by the battery 83 when the switch is open. In this case,no difficulties are encountered in synchronizing the pulsed magneticfield with the discharge. As can be seen from FIGS. 1 6, the pulverulentmass in part lies in a straight-line path between the electrodes.

It should be noted that the pulverulent mass can be held in the regionof the discharge or fusible wire by a loose-bonding agent (e.g., asynthetic resin) admixed with the powder, or by a retaining tube orstructure of a synthetic resin, paper or the like, with the lattermethod being preferred. It has been found that the presence of anextraneous substance like the resin tube does not affect the resin formof deposition which takes place whether such a tube is provided or theparticles are held in place by some other means. Thus it is alsoconceivable that the powder will completely fill the gap.

EXAMPLE I In an arrangement similar to that of FIG. 9, tungsten-carbidepowder 90 having a particle size of about 0.1 mm, was disposed in apolyethylene tube 91 having a wall thickness of 0.2 mm and an insidediameter of about 25 mm. A fusible copper wire 93 was passed axiallythrough the tube and had a diameter of about 0.12 mm. The wire, tube andworkpiece 94 had a length of about mm, the workpiece being composed ofSSSC carbon steel having a carbon content of 0.55 percent by weight. Thedetonation assembly was disposed at a distance 95 of mm from the surfaceof the workpiece in a chamber such as that shown in FIG. 3 and wasmaintained at a reduced pressure of 10 mm of mercury. After an electriccurrent was passed through the wire sufficient to melt it and cause adischarge with an energy of 3,000 joules, the deposit 96 upon thesurface of the workpiece was measured. The deposit was found to have athickness of 0.06 mm and to cover a width of the workpiece (i.e.,transverse to the major dimension of the wire) of about 50 mm, thecentral 25 mm of which was a continuous zone while the outer 12.5 mm oneach side was discontinuous as illustrated in FIG. 10, in which the ironsubstrate is seen to have particles of tungsten carbide disposedthereon. The magnification of 100 times shows the tungsten carbide ofFIG. 10 to be firmly bonded to the substrate although somewhatdiscontinuous. A section through the center region (enlarged 800 times)is shown in FIG. 11 from which it can be seen that the particles oftungsten carbide penetrate deeply into the surface of the substrate andare anchored firmly thereon, in a continuous layer. The roughness of thesurface at the central portion was found to be 3 microns (H,,,,,,). Thedischarge pulse had a duration of about 85 microseconds. The hardness Hy2,100 (on the Vickers scale).

When the arrangement of FIG. 8 was employed using a magnet having fourto live turns with a current ranging between 1,000 and 5,000 amperes,the layer of tungsten carbide was found to have the same hardness but athickness of OJ mm and a roughness of 3 4 microns H in both cases; thevelocity of the particles range between 100 m/sec and 5 km/sec.

In the system of FIG. 12, the discharge chamber is formed as a barrel100 whose mouth 101 is trained at the surface 102 of a substrate 103which can be either conductive or non-conductive, as described earlier.A gap 104 is provided around the zone of the surface 102 surrounded bythe barrel 100 to prevent pressure increases therewithin from reducingthe kinetic energy of the particles projected against the surface 103.At the other end of the barrel 100, an insulating block 113 receives aneedle-type electrode 112 which can be threaded into the barrel 100axially to a variable distance r from the region at which a hopper 114feeds the pulverulent material 105 into the barrel transversely. Thehopper 114 is provided with a feeding or metering mechanism 115 whosemotor 116 is driven intermittently by a timer.117 which also controls aswitch 109 in the supply circuit for the facing gun. The supply circuit106 comprises a direct-current source (shown as a battery 108) acrosswhich is bridged a capacitor 107 in series with a charging resistor 110.The distance I is adjusted in this embodiment until closure of switch109 will result in a discharge behind the mass of particles 105 whosepresence modifies the breakdown voltage which must be applied betweenthe needle 112 and the barrel across which the pulsing source 106 isconnected. When larger quantities of conductive powder are supplied inthe region of needle 112, the breakdown voltage is reduced and rapidpulses can be supplied so that a train of discharges at a repetitionfrequency determined by the timer 117 and synchronized with the particlefeed means can drive these particles against the surface 102. Ingeneral, the discharge takes place rearwardly of the particle mass 105and among these particles to partially ionize them, strip their oxidefilms and effect direct transfer of kinetic energy to the particles. Itwill also be understood that the timer means need not be used inasmuchas closure of switch 109 will apply a given potential between the needle112 and the barrel 100 and the firing of the discharge can be initiatedeither by advancing the needle 112 or by introducing a sufficientlylarge mass of the particles 105.

In the modification of FIG. 13, the barrel 200, trained upon thesubstrate 203 with a clearance 204 to prevent excess static pressurebuildup in the barrel, is provided with feed means including a supplyroll 219 for a foil 220 of a conductive material. The chamber 221 isformed at least in part by a barrel portion 222 electrically insulatedfrom a needle 212 which can be advanced by a motor 223 or hydraulicmeans as illustrated in greater detail in FIG. 14. A pulse source suchas that illustrated at 106 in FIG. 12 can be connected across the needle212 and a basket-shaped counterelectrode 224 just behind the foil 220.Thus the foil 220 constitutes a frangible diaphragm sealing the open endof chamber 221 and carrying a mass of, for example, nonconductive orpartially nonconductive particles 205. Upon discharge across the gapbetween the needle 212 and the counter-electrode 224, the shock wavedestroys the diaphragm 220 and propagates the particles 205 against thesurface 203 to form a coherent layer 225 thereon. It will be understoodthat the counterelectrode 224 can be omitted and the correspondingterminal of the pulse source 206 connected to the foil 220 so that thelatter may serve as the counterelectrode as illustrated in FIG. 13A.

EXAMPLE II Using an apparatus of the type illustrated in FIG. 13, thefoil 220 was a nickel-and-metal foil having a thickness of approximately0.006 mm. The particle mass was constituted by equal proportions of300-Mesh tungsten carbide and 600-Mesh synthetic diamond. Five grams ofthe particle mixture were placed upon the foil and a discharge with anenergy of 8,000 joules was applied at the needle 212. The workpiece wasa 0.55 percent by weight carbon steel (855C) and the coated, surface wasa distance of 12 mm from the foil. About 4 grams of the particles werefound to be strongly adherent to the workpiece. Corresponding resultswere obtained when the particles were composed of silicon carbide,aluminum nitride, boron nitride and titanium carbide. When the workpiecewas replaced by an aluminum foil it was found that deposition oftitanium and tantalum particles was readily carried out with the samedischarge energy and device.

In a modification of the system of Example II, the particles specifiedtherein were mixed with a binder to form a disk which was placed uponthe foil 220. Binders tested for this purpose included cellulosepropionate, paraoxybenzaldehyde, allyl-alcohol resin and hard rubber. Inall cases the binder was present in an amount just sufficient to holdthe mass together and it was found that the shock wave resulted in apenetration of the particles into the body as individual and discreteunits in spite of their bound state prior to the discharge. Penetrationof the particles was improved by incorporating stoichiometricequivalents of chromic oxide and of reducing binders of the characterdescribed.

In the system of FIG. 14, the barrel 300 extends into a coating chamber330 lined with a sound-damping elastomeric material 331 such as foamrubber. The chamber 330 is vented through a muffler 332 and is providedwith a cross-feed support 333 for the workpiece 303. The cross feedincludes spindles 334 and 335 for longitudinal and transversedisplacement of the workpiece 303. The upper part of the barrel 322 isseparated from the lower portion 300 by insulating spacers 336 uponwhich the foil 320 is mounted. The foil carries the mass 305 ofparticles and is here disposed within the chamber 321 in which the sparkis generated. The needle 312 passes through an insulating bushing 313and is connected with a supply network 306 of the type illustrated inFIG. 12. The firing control of the system is here regulated by ahydraulic motor 323 whose piston 337 is connected with the needle 312for hydraulically advancing same. A distributing valve 338 in circuitwith a hydraulic pump 339 and a reservoir 340 provides the necessaryregulation of a hydraulic device 323. When the electrode needle 312approaches the foil 320 sufficiently, a discharge results in rupture ofthe foil diaphragm and the propagation of the particles against theworkpiece 303. The discharge can also be initiated by the operation of acompressed-air source 341 designed to blow a stream of air into thechamber 321 and stir the particles therein to effect a breakdown betweenthe electrode and the foil.

In FIG. the device 400 comprises a fluid receptacle 401 which itselfconstitutes the piston of a hydraulic cylinder 457. Inlet and outlettubes 420 and 419 circulate the liquid medium 404 within vessel 401 viaa filter. A piston 439 is slidably displaceable within vessel 401 and isprovided with an insulating lining 439a. Piston 439 carries a deposit408 of electrode material and thus constitutes one of the electrodesforming the spark gap 405, the other electrode being a rod or wire 415adapted to be fed into vessel 401 by rollers 413 in response to analteration in the size of the spark gap. A vibratile bar 439", whoseresonant frequency is approximately equal to the resonant frequency ofthe discharge across gap 405, connects piston 439 with a plate 439' forcompression of a conductive powder 440 retained within the cavity 437 ofan electrically insulating sleeve 437' which is reinforced by ribs 455and mounted upon the metal plate 456. The two-position valve 458 isconnected in series with the hydraulic cylinder 457 and is supplied by alow-pressure conduit 459 and a high-pressure conduit 460, valve 458being operated by a control circuit 462 in response to the voltage dropacross the mass of particles 440; a battery 461, in series with thecontrol circuit 462, provides the necessary current for circuit 462. Thedischarge energy is supplied by a capacitor 441 connected between plate456 and electrode 415, capacitor 441 being bridged by a battery 443 incircuit with an inductance 444 and a resistance 444'. Vessel 401 isprovided with an annular recess 452, normally blocked by piston 439,which communicates with a high-pressure accumulator 453.

When conductive particles are employed, capacitor 441 discharges todevelop simultaneous sparks at gap 405 and through the particle mass,thereby forming conductive bridges among the particles. The shock wavewithin vessel 401 rebounds against the piston 439 so that the force ofthis piston compresses the conductive powder at the conclusion of theelectrical discharge. Simultaneously, control 462 senses the decreasedvoltage drop across the mass of particles and energizes valve 458 to cutoff the low-pressure fluid supply to cylinder 457, which formerlydisplaced vessel 401 to follow the shrinkage of the particle mass, andcut in the high-pressure conduit. The conductive powder, now sinteredinto a porous mass but still in a plastic state, is thus subjected tothe additive pressure of source 460 and the pressure wave within vessel401. When nonconductive particles are used, capacitor 441 is connectedto the piston 439 as indicated by the dot-dash conductor 463 whereuponthe pressure of the discharge at gap 405 is applied to the particleswithout initial formation of bridges across them by electricaldischarge.

EXAMPLE III A mass of polytetrafluorethylene particles of ZOO-Mesh aredisposed in a nonconductive sleeve having a diameter of 15 mm and alength of 2 cm. Light pressure was applied at hydraulic cylinder 457 tocompress the particles (approximately 1 kg/cm) while a discharge insilicone oil within vessel EXAMPLE IV The procedure of Example 111 wasfollowed, except that nickel particles and a spark energy of 5,000joules was used between plate 456 and electrode 415. The pressureapplied by cylinder 457 to the particles was 1 kglcm this pressure beingfollowed upon reduction of the voltage drop across the mass of particlesto a value of 500 kglcm the discharge terminating concurrently with theincrease in pressure. The resulting body had the density of greater thanpercent of that of the solid mass.

The invention described and illustrated is believed to admit of manymodifications within the ability of persons skilled in the art, all suchmodifications being considered within the spirit and scope of theappended claims.

lclaim:

1. A method of forming coherent bodies from a pulverulent material,comprising the steps of:

a. juxtaposing a pair of electrodes in spaced relationship with oneanother for producing a shock wave upon energization;

b. providing a mass of particulate material in close proximity to atleast one of said electrodes and in substantially a straight-line pathbetween them for activation by said electrodes and propulsion of saidmaterial by said shock wave upon the formation of a discharge acrosssaid electrodes, said mass being at least in part within the discharge;

c. positioning a substrate ahead of said mass of material in saiddirection whereby a surface of said substrate is disposed forimpingement of said particulate material thereon upon the generation ofa spark discharge between said electrodes while maintaining anoncombustible gaseous environment between said electrode and saidsubstrate; and

d. energizing said electrodes by applying an electric pulse thereto togenerate an impulsive spark discharge between said electrodes and propelsaid material in the direction of said substrate and form a coherentmass of said material thereon.

2. A method of forming coherent bodies fom a pulverulent material,comprising the steps of:

a. juxtaposing a pair of electrodes in spaced relationship with oneanother for producing a shock wave upon energization;

b. providing a mass of particulate material in close proximity to atleast one of said electrodes for activation thereby and propulsion ofsaid material by said shock wave upon the formation of a dischargeacross said electrodes, said mass being at least in part within thedischarge;

c. positioning a substrate ahead of said mass of material in saiddirection whereby a surface of said substrate is disposed forimpingement of said particulate material thereon upon the generation ofa spark discharge between said electrodes while maintaining anoncombustible gaseous environment between said electrode and saidsubstrate; and

d. energizing said electrodes by applying an electric pulse thereto togenerate an impulsive spark discharge between said electrodes and propelsaid material in the direction of said substrate and form a coherentmass of said material thereon, said mass being admixed with a binder andforming an explodable sheath surrounding said electrodes prior to thegeneration of said discharge.

3. A method of forming coherent bodies from a pulverulent material,comprising the steps of:

a. juxtaposing a pair of electrodes in spaced relationship with oneanother for producing a shock wave upon energization;

b. providing a mass of particulate material in close proximity to atleast one of said electrodes for activation thereby and propulsion ofsaid material by said shock wave upon the formation of a dischargeacross said electrodes, said mass being at least in part within thedischarge;

c. positioning a substrate ahead of said mass of material in saiddirection whereby a surface of said substrate is disposed forimpingement of said particulate material thereon upon the generation ofa spark discharge between said electrodes while maintaining anoncombustible gaseous environment between said electrode and saidsubstrate; and

d. energizing said electrodes by applying an electric pulse thereto togenerate an impulsive spark discharge between said electrodes and propelsaid material in the direction of said substrate and form a coherentmass of said material thereon, said electrodes being initially bridgedby an electrically destructible conductor adapted to explode upon theapplication of an electrical pulse, said method further comprising thestep of explosively destroying said conductor by incipient passagetherethrough of the electrical pulse forming said discharge to form anelectrode gap sustaining said discharge during the remainder of saidpulse, said mass being at least in part created by destruction of saidconductor.

4. A method of forming coherent bodies from a pulverulent material,comprising the steps of:

a. juxtaposing a pair of electrodes in spaced relationship with oneanother to form an impulse generator;

b. disposing a mass of pulverulent material in force-receivingrelationship with said generator;

c. positioning a substrate in force-receiving relationship with saidgenerator whereby a surface of said substrate is disposed forimpingement of said pulverulent material thereon upon the generation ofa spark discharge between said electrodes, said particles beingsupported between at least one of said electrodes and said surface in alayer substantially parallel thereto upon a rupturable electricallyconductive diaphragm forming the other elect'rode; and

d. energizing said electrodes by applying an electric pulse thereto togenerate an impulsive spark discharge and shock wave between saidelectrodes of an intensity sufiicient to cause impingement of saidpulverulent material upon said substrate and to bond said pulverulentmaterial into a coherent mass on said substrate, said electrodes beingspacedly juxtaposed in a gaseous environment.

5. The method defined in claim 4 wherein said pulverulent material isconstituted as a relatively loose mass of particles.

6. The method defined in claim 4, further comprising the step of heatingsaid mass of pulverulent material independently of said spark dischargeto facilitate adhesion thereof to said substrate.

7. The method defined in claim 6 wherein said pulverulent material isheated prior to the impingement of said mass against said substrate byadmixing with said mass prior to said spark discharge at least onechemical compound capable of exothermic reaction in said mass, saidexothermic reaction being initiated at least in part by the saiddischarge.

8. The method defined in claim 4, further comprising the step ofdirecting the shock wave generated at said electrodes toward saidsurface by applying an electromagnetic force field to said discharge.

9. The method defined in claim 4 wherein a multiplicity of impulsivedischarges are generated at said electrodes, further comprising the stepof sweeping said generator over said surface during said multiplicity ofdischarges.

10. The method defined in claim 4, further comprising the step ofevacuating the space between said mass and said surface.

ll. A method of forming coherent bodies from a pulverulent material,comprising the steps of:

a. juxtaposing a pair of electrodes in spaced relationship with oneanother to form an impulse generator adapted to produce a shock wave;

b. positioning a substrate in force-receiving relationship with saidgenerator whereby a surface of said substrate is exposed to said shockwave and disposed for impingement of pulverulent material propelledthereby;

c. disposing a mass of a pulverulent material in force-receivingrelationship with said generator and in the form of a relatively loosemass of particles supported on a solid surface between at least one ofsaid electrodes and said surface in a layer substantially parallelthereto; and

d. energizing said electrodes by applying an electric pulse thereto togenerate an impulsive spark discharge between said electrodes at leastin part through said mass of pulverulent material and of an intensitysufficient to propel said particles against said substrate and to bondsaid pulverulent material into a coherent mass thereon, said electrodesbeing spacedly juxtaposed in a noncombustible gaseous environment.

12. The method defined in claim 11, further comprising the step ofheating said mass of pulverulent material independently of said sparkdischarge to facilitate the adhesion thereof to said substrate.

13. The method defined in claim 12 wherein said pulverulent material isheated independently of said spark discharge and prior to theimpingement of said mass against said substrate by admixing with saidmass prior to said spark discharge at least one chemical compoundcapable of exothermic reaction in said mass, said exothermic reactionbeing initiated at least inpart by the said discharge.

14. The method defined in claim 11, further comprising the step ofdirecting the shock wave generated at said electrodes toward saidsurface by applying an electromagnetic force field to said discharge.

15. The method defined in claim 11 wherein a multiplicity of impulsivedischarges are generated at said electrodes, further comprising the stepof sweeping said generator over said surface during said multiplicity ofdischarges.

16. The method defined in claim 11, further comprising the step ofevacuating the space between said mass and said surface.

17. An apparatus for forming coherent bodies from a particulatematerial, comprising:

an impulse generator provided with a pair of electrodes in spacedrelationship with one another;

means for disposing a mass of said particulate material in closeproximity to the electrodes of said generator;

circuit means for energizing said electrodes by applying thereto anelectric pulse to generate an impulsive spark discharge between saidelectrodes of an intensity sufficient to bond said particulate materialinto a coherent mass;

housing means open in the direction of a substrate adapted to receive alayer of said particulate material and containing said generator; and

solid support means for said particulate material in said housing meansbetween at least one of said electrodes and a surface of said substrateadapted to receive said layer.

18. An apparatus as defined in claim 17 wherein said electrodes areinitially bridged by a fusible conductor destructible upon incipientpassage of an electrical pulse to form a discharge gap between saidelectrodes.

19. An apparatus as defined in claim 17, further comprising means for atleast partially evacuating said housing means intermediate saidgenerator and said surface.

20. An apparatus as defined in claim 17, further comprising means forapplying an electromagnetic force field across the region between saidgenerator and said surface for directing the shock wave produced by saiddischarge.

21. An apparatus as defined in claim 17, further comprising means forshifting the electrode gap defined between said electrodes and saidsurface relatively in a direction parallel to the latter 22. Anapparatus for forming coherent bodies from a pulverulent material,comprising an impulse generator provided with a pair of electrodes inspaced relationship with one another; means for disposing a mass of saidpulverulent material in force-receiving relationship with saidgenerator; circuit means for energizing said electrodes by applyingthereto an electric pulse to generate an impulsive spark dischargebetween said electrodes of an intensity sufficient to bond saidpulverulent material into a coherent mass; housing means open in thedirection of a substrate adapted to receive a layer of said pulverulentmaterial and containing said generator; and support means for saidpulverulent material in said housing means between at least one of saidelectrodes and a surface of said substrate adapted to receive saidlayer, said support means being formed as a foil extending parallel tosaid surface and defining a frangible diaphragm between said generatorand said surface, said diaphragm retaining said mass.

23. An apparatus as defined in claim 22 wherein said foil iselectrically conductive and forms the other of said electrodes.

24. An apparatus as defined in claim 23 wherein said mass is disposed asa loose layer of conductive particles upon said foil between said oneelectrode and the foil, said apparatus further including means fordirecting a stream of gas against said mass to entrain said particlesand initiate said discharge.

25. An apparatus as defined in claim 22 wherein said foil forms a sleevefor a sheath of said pulverulent material surrounding said electrodes.

I? t III I

2. A method of forming coherent bodies fom a pulverulent material,comprising the steps of: a. juxtaposing a pair of electrodes in spacedrelationship with one another for producing a shock wave uponenergization; b. providing a mass of particulate material in closeproximity to at least one of said electrodes for activation thereby andpropulsion of said material by said shock wave upon the formation of adischarge across said electrodes, said mass being at least in partwithin the discharge; c. positioning a substrate ahead of said mass ofmaterial in said direction whereby a surface of said substrate isdisposed for impingement of said particulate material thereon upon thegeneration of a spark discharge between said electrodes whilemaintaining a noncombustible gaseous environment between said electrodeand said substrate; and d. energizing said electrodes by applying anelectric pulse thereto to generate an impulsive spark discharge betweensaid electrodes and propel said material in the direction of saidsubstrate and form a coherent mass of said material thereon, said massbeing admixed with a binder and forming an explodable sheath surroundingsaid electrodes prior to the generation of said discharge.
 3. A methodof forming coherent bodies from a pulverulent material, comprising thesteps of: a. juxtaposing a pair of electrodes in spaced relationshipwith one another for producing a shock wave upon energization; b.providing a mass of particulate material in close proximity to at leastone of said electrodes for activation thereby and propulsion of saidmaterial by said shock wave upon the formation of a discharge acrosssaid electrodes, said mass being at least in part within the discharge;c. positioning a substrate ahead of said mass of material in saiddirection whereby a surface of said substrate is disposed forimpingement of said particulate material thereon upon the generation ofa spark discharge between said electrodes while maintaining anoncombustible gaseous environment between said electrode and saidsubstrate; and d. energizing said electrodes by applying an electricpulse thereto to generate an impulsive spark discharge between sAidelectrodes and propel said material in the direction of said substrateand form a coherent mass of said material thereon, said electrodes beinginitially bridged by an electrically destructible conductor adapted toexplode upon the application of an electrical pulse, said method furthercomprising the step of explosively destroying said conductor byincipient passage therethrough of the electrical pulse forming saiddischarge to form an electrode gap sustaining said discharge during theremainder of said pulse, said mass being at least in part created bydestruction of said conductor.
 4. A method of forming coherent bodiesfrom a pulverulent material, comprising the steps of: a. juxtaposing apair of electrodes in spaced relationship with one another to form animpulse generator; b. disposing a mass of pulverulent material inforce-receiving relationship with said generator; c. positioning asubstrate in force-receiving relationship with said generator whereby asurface of said substrate is disposed for impingement of saidpulverulent material thereon upon the generation of a spark dischargebetween said electrodes, said particles being supported between at leastone of said electrodes and said surface in a layer substantiallyparallel thereto upon a rupturable electrically conductive diaphragmforming the other electrode; and d. energizing said electrodes byapplying an electric pulse thereto to generate an impulsive sparkdischarge and shock wave between said electrodes of an intensitysufficient to cause impingement of said pulverulent material upon saidsubstrate and to bond said pulverulent material into a coherent mass onsaid substrate, said electrodes being spacedly juxtaposed in a gaseousenvironment.
 5. The method defined in claim 4 wherein said pulverulentmaterial is constituted as a relatively loose mass of particles.
 6. Themethod defined in claim 4, further comprising the step of heating saidmass of pulverulent material independently of said spark discharge tofacilitate adhesion thereof to said substrate.
 7. The method defined inclaim 6 wherein said pulverulent material is heated prior to theimpingement of said mass against said substrate by admixing with saidmass prior to said spark discharge at least one chemical compoundcapable of exothermic reaction in said mass, said exothermic reactionbeing initiated at least in part by the said discharge.
 8. The methoddefined in claim 4, further comprising the step of directing the shockwave generated at said electrodes toward said surface by applying anelectromagnetic force field to said discharge.
 9. The method defined inclaim 4 wherein a multiplicity of impulsive discharges are generated atsaid electrodes, further comprising the step of sweeping said generatorover said surface during said multiplicity of discharges.
 10. The methoddefined in claim 4, further comprising the step of evacuating the spacebetween said mass and said surface.
 11. A method of forming coherentbodies from a pulverulent material, comprising the steps of: a.juxtaposing a pair of electrodes in spaced relationship with one anotherto form an impulse generator adapted to produce a shock wave; b.positioning a substrate in force-receiving relationship with saidgenerator whereby a surface of said substrate is exposed to said shockwave and disposed for impingement of pulverulent material propelledthereby; c. disposing a mass of a pulverulent material inforce-receiving relationship with said generator and in the form of arelatively loose mass of particles supported on a solid surface betweenat least one of said electrodes and said surface in a layersubstantially parallel thereto; and d. energizing said electrodes byapplying an electric pulse thereto to generate an impulsive sparkdischarge between said electrodes at least in part through said mass ofpulverulent material and of an intensity sufficient to propel saidparticles against said substrate and to bond said pulverulent materialinto a coherent mass thereon, said electrodes being spacedly juxtaposedin a noncombustible gaseous environment.
 12. The method defined in claim11, further comprising the step of heating said mass of pulverulentmaterial independently of said spark discharge to facilitate theadhesion thereof to said substrate.
 13. The method defined in claim 12wherein said pulverulent material is heated independently of said sparkdischarge and prior to the impingement of said mass against saidsubstrate by admixing with said mass prior to said spark discharge atleast one chemical compound capable of exothermic reaction in said mass,said exothermic reaction being initiated at least in part by the saiddischarge.
 14. The method defined in claim 11, further comprising thestep of directing the shock wave generated at said electrodes towardsaid surface by applying an electromagnetic force field to saiddischarge.
 15. The method defined in claim 11 wherein a multiplicity ofimpulsive discharges are generated at said electrodes, furthercomprising the step of sweeping said generator over said surface duringsaid multiplicity of discharges.
 16. The method defined in claim 11,further comprising the step of evacuating the space between said massand said surface.
 17. An apparatus for forming coherent bodies from aparticulate material, comprising: an impulse generator provided with apair of electrodes in spaced relationship with one another; means fordisposing a mass of said particulate material in close proximity to theelectrodes of said generator; circuit means for energizing saidelectrodes by applying thereto an electric pulse to generate animpulsive spark discharge between said electrodes of an intensitysufficient to bond said particulate material into a coherent mass;housing means open in the direction of a substrate adapted to receive alayer of said particulate material and containing said generator; andsolid support means for said particulate material in said housing meansbetween at least one of said electrodes and a surface of said substrateadapted to receive said layer.
 18. An apparatus as defined in claim 17wherein said electrodes are initially bridged by a fusible conductordestructible upon incipient passage of an electrical pulse to form adischarge gap between said electrodes.
 19. An apparatus as defined inclaim 17, further comprising means for at least partially evacuatingsaid housing means intermediate said generator and said surface.
 20. Anapparatus as defined in claim 17, further comprising means for applyingan electromagnetic force field across the region between said generatorand said surface for directing the shock wave produced by saiddischarge.
 21. An apparatus as defined in claim 17, further comprisingmeans for shifting the electrode gap defined between said electrodes andsaid surface relatively in a direction parallel to the latter.
 22. Anapparatus for forming coherent bodies from a pulverulent material,comprising an impulse generator provided with a pair of electrodes inspaced relationship with one another; means for disposing a mass of saidpulverulent material in force-receiving relationship with saidgenerator; circuit means for energizing said electrodes by applyingthereto an electric pulse to generate an impulsive spark dischargebetween said electrodes of an intensity sufficient to bond saidpulverulent material into a coherent mass; housing means open in thedirection of a substrate adapted to receive a layer of said pulverulentmaterial and containing said generator; and support means for saidpulverulent material in said housing means between at least one of saidelectrodes and a surface of said substrate adapted to receive saidlayer, said support means being formed as a foil extending parallel tosaid surface and defining a frangible diaphragm between said generatorand said surface, said diaphragm retaining said mass.
 23. An apparatusas deFined in claim 22 wherein said foil is electrically conductive andforms the other of said electrodes.
 24. An apparatus as defined in claim23 wherein said mass is disposed as a loose layer of conductiveparticles upon said foil between said one electrode and the foil, saidapparatus further including means for directing a stream of gas againstsaid mass to entrain said particles and initiate said discharge.
 25. Anapparatus as defined in claim 22 wherein said foil forms a sleeve for asheath of said pulverulent material surrounding said electrodes.